This invention relates to down-hole probe assemblies for use in conditions of high vibration or shock, such as are encountered within the bottomhole assembly of a rotating drill string during drilling.
During downhole measurement-while-drilling (MWD) one or more measurement probes are located inside the drill collar portion of the drill string close to the drill bit, and there is a risk that such measurement probes will suffer damage or that the measurements taken will be compromised by the high levels of vibration or shock to which the probes are subjected in use.
One form of probe which is used is the gamma ray detector probe which detects the gamma radiation received from radioactive elements in the formations penetrated by the borehole being drilled, for the purpose of producing a gamma ray log against depth for use in formation analysis. Such gamma ray detector probes generally comprise a scintillation counter having a gamma ray scintillator crystal and a photomultiplier tube joined at an optical interface formed, for example, of silicone grease. The integrity of the optical interface between the crystal and the photomultiplier tube can be affected by vibrations and this can seriously compromise the performance of the scintillation counter.
It is an object of the invention to improve the mounting of a scintillation counter or other vibration-sensitive inner unit of a downhole probe assembly so as to protect the unit against the effects of vibration.